Thin films of hyperbranched polyesters were studied in dry state and in aqueous buffer solution regarding their swelling behaviour and protein adsorption potential. The influence of the degree of branching, the backbone structure, flexibility as well as the polarity was varied. By changing the backbone structure from aromatic, aromatic-aliphatic to aliphatic the surface properties can be controlled from protein active to protein repelling. The higher adsorption potential observed in comparison to linear polyesters is the result of the large amount. of end groups allowing the formation of hydrogen bonds, and the larger swellability of the more flexible linear polymers. The protein adsorption process was studied intensively by in-situ spectroscopic ellipsometry. Different approaches towards a proper optical model for the vis-ellipsometry data evaluation for the determination of the correct layer thickness and refractive index are discussed. IR-ellipsometric measurements using a new in-situ cell gave the full chemical evidence for the formation of thin protein adsorption layer on the polymer films in the aqueous buffer environment. (C) 2008 Elsevier B.V. All rights reserved.